Graham,
Like Dan, I read what you wrote a few times over, to put everything in order before I say anything.
Ok here goes....
Firstly I dont believe he should be saying anything to you about the speed that you work as you are being paid for the m2 and your not on a day rate. Everyone works at diff rates, I have one lad whose work is outstanding, but he gets gyp of the others because he takes his time about. What they dont realise is that this lad never gives me any problems with snagging works, and I have never had to pull him on anything hes ever done for me. Next time plumber boy says anything, one simple thing to say, quality not quantity.
Secondly stop worrying yourself about how long your taking to do the works. Your going to beat yourself up over doing something right? Push that thought right out of your mind.
Now Im going to tackle the money.... I was shocked to see the rate hes paying you, and horrified when I read that he makes you wait for him to get paid. Sorry but this is a no no. Plumber boy should make arrangements that his guys get paid every two weeks. I would never ever dream of making anyone of my lads wait till I get paid. I work to a tight budget, and my priority is that all my lads get paid fortnightly. Even if that means I pay myself late that particular month, they get paid without question. I would never dream of pocketing the money of the last job and make them wait.
I really feel you should speak with him tomorrow, go down the cafe for a cuppa, and explain that firstly, the work recently has been a lot more complicated than you anticipated, and for all the extra cutting etc, you should be getting a much better rate than he is paying right now. You seriously think plumber boy is charging the client the same for ceramics as marble and stone....I think not! If hes underpriced the job, how is that your problem? he submitted the price to the client, if he messed up, let him take the knock not expect his team to. If he doesnt see it your way and stands his ground, then you tell him that you will finish of the works in the room your working in and will leave on Friday as you dont want to fall out about it. And for gawds sake make sure you tell him you want your money on Friday as fair is fair. Be make sure to tell him though you dont want to go elsewhere for work, but you cant work for these rates.
I just read your post again, and Im a bit fuming now....how dare he tell you that you cant have time of to spend them with your kids...the selfish piece of work. Go let him sulk, he should have made arrangements for cover.
Keep a cool head, and stand your ground, and dont be bullied.
Good luck hun, and let us know how you get on
Lynn